In the production of color photographic images, it is necessary to remove the silver image which is formed coincident with the dye image. This can be done by oxidizing the silver by means of a suitable oxidizing agent, commonly referred to as a bleaching agent, in the presence of halide ion, followed by dissolving the silver halide so formed in a silver halide solvent, commonly referred to as a fixing agent. Alternatively, the bleaching agent and fixing agent can be combined in a bleach-fixing solution and the silver removed in one step by use of such solution.
A wide variety of bleaching agents are known for use in photographic processing, for example, ferricyanide bleaching agents, persulfate bleaching agents, dichromate bleaching agents, permanganate bleaching agents, ferric chloride, and water-soluble quinones.
It is particularly well known to use a ferric complex of an aminopolycarboxylic acid as a bleaching agent in photographic color processing. Such complexes are used in both bleaching compositions and bleach-fixing compositions. A very large number of different compounds of the aminopolycarboxylic acid class are disclosed in the prior art as being useful photographic bleaching agents. However, the usual commercial practice is to use an ammonium or alkali metal salt of a ferric complex of ethylenediaminetetraacetic acid (EDTA) or of a ferric complex of propylenediaminetetraacetic acid (PDTA).
Among the numerous patents describing the use of ferric complexes of aminopolycarboxylic acids in bleaching and/or bleach-fixing baths are U.S. Pat. Nos. 3,241,966, 3,615,508 and 3,767,401 and British patents 1,365,453, 1,392,163, and 1,394,357.
The prior art indicates that ferric complexes of an aminopolycarboxylic acid can be employed in photographic bleaching solutions in the form of ammonium or alkali metal salts, such as sodium or potassium salts. However, the use of ammonium salts is undesirable from an environmental standpoint and the use of sodium salts provides an undesirably sluggish bleaching action. Thus, it is especially advantageous to utilize photographic bleaching solutions which comprise a potassium salt of a ferric complex of an aminopolycarboxylic acid as the bleaching agent, since these solutions are both fast-acting and environmentally acceptable.
A serious problem is encountered, however, in utilizing a photographic bleaching solution containing a potassium salt of a ferric complex of an aminopolycarboxylic acid. In particular, when such a bleaching solution is used to bleach a photographic element containing a low pKa, high-activity, yellow-dye-forming coupler, a significant and highly undesirable increase in blue D.sub.min results from bleach induced dye formation. The term "bleach induced dye formation" refers to reaction between the coupler and oxidized developing agent that takes place in the bleaching solution and leads to the generation of image dye and thus to stain formation. Formation of yellow image dye in this manner and resulting increase in blue D.sub.min is particularly acute with the low pKa, high-activity, yellow-dye-forming couplers that are used in many photographic films. The problem also occurs with the other image layers, but to a much lesser extent.
It is toward the objective of providing an improved photographic bleaching solution that is fast-acting, and environmentally acceptable--and sufficiently resistant to the phenomenon of bleach induced dye formation that it is highly useful with photographic elements containing a low pKa, high activity, yellow-dye-forming coupler--that the present invention is directed.